This invention relates to refractories having high durability for use in the casting of molten metals and, more particularly, to refractory sliding nozzle plates formed of the refractories.
Refractory sliding nozzle plates which are employed as components in the so-called sliding nozzle system (the system will be referred to as "SN system" hereinafter). The SN system includes a plurality of refractory brick or refractory plates having nozzle openings through which a flow of molten metal passes and the plates slide relative to each so as to open and close their nozzle openings. The system has been widely utilized as an effective means for controlling the flow rate of molten metal and especially, the SN system has been most employed as a flow rate control means for a flow of molten pig iron or steel in the iron and steel industry where refractory parts in the system have been subjected to more and more severe operation conditions. The SN system generally comprises three principal components, that is, an upper refractory nozzle, at least two or upper and lower slidable refractory plates and a lower refractory nozzle which are designed to perform their respectively expected functions. Among the principal components of the SN system, the refractory plates function to control the flow rate of molten metal and are required to have high surface preciseness, wear-resistance and refractoriness. The SN system refractory plates are subjected to physical actions such as abrupt thermal impact and wear from the molten metal flow as well as physical and chemical corrosive actions from the molten metal and slag and thus, the refractory plates are generally required to possess spalling and corrosion resistances and sufficient strength.
Hitherto, efforts have been made to coordinately impart the SN system refractory plates with the above-mentioned properties and in most cases, as the principal material of the SN system refractory plate, alumina is fired to a temperature within the range of 1300.degree.-1800.degree. C. to provide oxide ceramic bond. In order to improve the durability of the SN system refractory plate formed of such oxide ceramic bond, of late, such a plate has been impregnated with coal tar pitch or the like, but the refractory plate impregnated with coal tar pitch has the disadvantage that the volatile matter or matters in the coal tar pitch emit smoke and offensive smell to substantially pollute the environment when the refractory plate is used for its intended purpose. Of late, in order to eliminate the disadvantage inherent in the oxide ceramic bond impregnated with coal tar pitch or the like, a non-fuming alumina carbon which is not impregnated with coal tar pitch or the like, but maintains its high durability has been developed as the material of the SN system refractory plate. The non-fuming alumina carbon exhibits a durability higher than that of the oxide ceramic bond and has been more widely employed than the latter. The present invention is directed to not only such non-fuming alumina carbon refractories for the SN system plates, but also refractories impregnated with coal tar pitch and/or resins for the SN system refractory plates within the scope of the present invention.
The SN system refractory plates are designed to be heated about the nozzle openings therein by the heat from a flow of molten metal such as steel and employed with an uneven temperature distribution therein and thus, when the refractory plates have a high coefficient of thermal expansion, the upper and lower refractory plates tend to lose their closely contacting relationship which would result in the acceleration of surface roughening and the occurrence and/or enlargement of a crack or cracks in the nozzle openings in the plates. Therefore, in order to improve the durability of the SN system refractory plates, it is considered that the lowering of coefficient of thermal expansion of the refractory plates is an effective means. The requirement for lowering the coefficient of thermal expansion for the SN system refractory plates is also applicable to the long nozzle connecting between the laddle, tandish and mould and to the immersion nozzle in the SN system.